Lecture 8: Planar X-ray Flashcards

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1
Q

What is the typical energy range of x-rays?

A

100 eV to 100 keV

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2
Q

What is the difference between hard x-rays and soft x-rays?

A

Hard: greater than 10 keV energy; penetrate liquids and some solids.
Soft: less than 10 keV energy; hardly penetrate matter.

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3
Q

Why are hard x-rays used in diagnostic imaging?

A

They can go through the patient to be detected on the other side.

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4
Q

How are planar x-rays used to generate images?

A

Different tissues have different levels of absorption which changed the amounts of radiation that passes through the body to be imaged.

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5
Q

What part of the body absorbs the most x-rays? Why?

A

Bone as it is very dense.

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6
Q

Define the linear attenuation coefficient

A

A constant that characterises how easily a volume of material can be penetrated by a beam of light, sound, particles, or other energy/matter.

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7
Q

What is the differential form of the intensity equation?

A

dI = change in beam intensity
I = original beam intensity
dx = incremental distance
n = number of atoms per unit volume
σ = constant related to the probability of scattering

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8
Q

What is the equation for beam intensity when the linear attenuation coefficient is constant?

A

I(x) = intensity
I_0 = initial beam intensity
n = number of atoms per unit volume
σ = constant related to probability of scattering
x = position
µ = linear attenuation coefficient

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9
Q

What is the general equation for the linear attenuation coefficient?

A

I(x) = intensity
I_0 = initial beam intensity
dx = incremental distance
µ = linear attenuation coefficient

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10
Q

What two things does the linear attenuation coefficient vary as a function of?

A
  • Tissue density
  • X-ray intensity
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11
Q

What are the three ways in which x-rays interact with matter?

A
  • Photoelectric effect
  • Compton scattering
  • Pair production
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12
Q

Define the photoelectric effect

A

When a photon interacts with an electron the electron is emitted from the atom as a photoelectron and the photon is absorbed.

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13
Q

What is the relation that described the contribution of the photoelectric effect to the linear attenuation coefficient?

A

µ = linear attenuation coefficient
ρ = density
Z = atomic number
E = energy

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14
Q

When is the photoelectric effect most important?

A

At low energies.

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15
Q

Define Compton scattering

A

When a photon is scattered by a weakly bound electron the electron is ejected and photon energy is reduced by an amount (depending on the scattering angle).

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16
Q

What is the relation that described the contribution of Compton scattering to the linear attenuation coefficient?

A

µ = linear attenuation coefficient
ρ = density

17
Q

Which applications depend most on Compton scattering? Why?

A

Diagnostic applications as the linear attenuation coefficient varies mainly with density.

18
Q

Define pair production

A

When a photon interacts with the nucleus, forming an electron/positron pair. This only occurs for energies greater than 1.02 MeV and isn’t relevant for diagnostic x-rays.